The present invention relates to water treating systems. Particularly, the invention relates to a method and apparatus for the utilization of wastewater.
The problem of scale is inherent to all systems in which there is a flow of water that contains any of Ca++ and Mg++ ions together with any of OH., CO3xe2x95x90, HCO3xe2x80x94, SiO3xe2x95x90, SO4xe2x95x90 or PO4xe2x95x90. Under certain temperature and pH conditions, carbonates, silicates, sulfates, phosphate and hydroxide salts precipitate and cause blockage of nozzles, reduction of cross-section area of pipes, heat insulation and underdeposit corrosion. The well-known methods of removing scale from aqueous liquids are reverse osmosis and ion exchange. Another method for removing scale is direct current (DC) electrolysis. U.S. Pat. No. 4,384,943 discloses a method of fluid treatment which comprises applying DC current to aqueous liquids.
Electrolytic treatment of aqueous fluids to produce biocides is well known in the art. For example, U.S. Pat. No. 4,384,943 describes such a treatment which comprises maintaining in the fluid a compound that is electrochemically decomposable to yield bromine, chlorine or iodine and/or by decomposing water to produce biocidally active O2 or O3 oxidants.
U.S. Pat. No. 6,424,032 describes a method of treating water using innocuous chemicals for the treatment of microorganisms, or employing ultraviolet light or electrolysis in order to destroy microorganisms.
In a copending patent application by the same applicant hereof, WO 99/16715, the description of which is entirely incorporated herein by reference, there is described a method of treatment of aqueous media, comprising applying to said aqueous medium in an electrolytic cell an electrical direct current of a magnitude and at a flow-rate of the liquid in said electrolytic cell such that a combined effect of scale removing and disinfecting is achieved.
WO 99/16715 further provides an aqueous fluid treatment device for scale removing and disinfecting comprising an electrolytic cell operated by a direct current source, said electrolytic cell being adapted to allow an aqueous medium to circulate therethrough.
The term xe2x80x9cdisinfectingxe2x80x9d, as used herein, means destroying various types of microorganisms to the extent that this prevents the formation of biological fouling, and disinfection of drinking water or of water for use in bathing.
Since scale removing and scale preventing are related processes, each of the terms xe2x80x9cscale removingxe2x80x9d and xe2x80x9cscale preventingxe2x80x9d herein mean both scale removing and scale preventing.
However, all prior art methods and apparatus suffer from the severe drawback that they require, in order to operate, to utilize substantially non-polluted water, since the addition of impure water leads to operational problems, to high purifying chemicals demand, and to undesirable disposal of waste. Particularly undesired, according to the known art, is the use of water containing organic contaminants. Thus, essentially clean water is wasted, e.g., for operating cooling towers, while waste water from the tower and other industrial or urban sources has to be treated prior to disposal, in an expensive manner.
Cooling towers cannot use water with a high content of organic and biologic materials, since the resulting biofouling, developing as a matter of hours, initially reduces the efficiency of the tower, and eventually may lead to a clogging of the tower""s fill.
It has now been surprisingly found, and this is an object of the invention, that it is possible to utilize waste water to operate cooling tower and related equipment, including high organic and biological loads, thereby obtaining the dual result of exploiting less expensive water for operation, and reducing the emission of industrial and urban wastes. For the purpose of illustrating the invention, it could be mentioned that a cooling tower operating according to invention, which utilizes a make-up of 330 m3 of wastewater every day (e.g., from an industrial plant), reduces the emission of waste to a mere 30 m3 a day, by employing the present invention, the remaining water being evaporated. Thus, instead of treating large volumes of diluted wastewater, only small amounts (less than 10%) of concentrated waste is to be treated. This also results in a lower demand for wastewater treatment volumes.
It is an objet of the invention to provide a method for the utilization of wastewater, which overcomes the aforementioned drawback of the prior art.
It is another object of the invention to provide apparatus, particularly cooling tower apparatus that employs impure water.
Other objects of the invention will become apparent as the description proceeds.
In one aspect the invention is directed to a method of operating a cooling tower, comprising feeding to said cooling tower a make-up stream of water containing organic and/or biological contaminants, causing a side stream taken from the recirculating stream to pass through an electrolytic cell, removing solids precipitating by the action of said cell, and remixing said treated side stream with the main stream, before feeding them to the cooling tower.
According to a preferred embodiment of the invention, the recirculating water is further oxidized by the addition of an oxidizing material. Addition of the oxidant is preferably, but non-limitatively, effected in an amount suitable to maintain the Redox potential at the inlet of the cooling tower in the range of about 300-400 mV. Ilustrative and non-limitative examples of suitable oxidants include NaClO, TCCA, BCDMH, Br2 and Cl2.
According to another preferred embodiment of the invention, a non-oxidizing biocide is added to the recirculating stream as an aid in the prevention of biofouling. Preferably, but non-limitatively, the biocide is added when the Redox potential decreases to a value of about 200 mV or less due to high organic load. Illustrative and non-limitative examples of suitable non-oxidizing biocides include phenolic biocides, quaternary amines, triazolin, DBNPA (dibromonitrilproprionamide), MIT (methyl izothiazolinone) or MBT (Methylene Bis Thiocyanate).
As will be appreciated by the skilled person, the COD of the make-up stream, at high COD values, is typically not less than about 600 and only at peak times reaches over 2,000 ppm. Normally, however, the COD of the make-up stream is maintained between 500 and 1000 ppm. As will be appreciated by the skilled person, the invention is not limited to CODs of 500 and above, and can of course operate at lower CODs. However, the advantages attained by the invention are even more enhanced when high CODs, or 500 or more are used, which are unheard of in the art of operating cooling towers.
According to a preferred embodiment of the invention, the Redox potential of the stream entering the cooling tower is in the range 300-400 mV.
In another aspect the invention is directed to a cooling tower system comprising, in combination with suitable inlets and outlets:
a cooling tower;
a heat-exchanger;
an electrolytic cell; and
at least one filter.
In a further aspect, the invention is directed to a method for concentrating waste water, comprising feeding said waste water to a cooling tower, causing a side stream taken from the recirculating stream to pass through an electrolytic cell removing solids precipitating by the action of said cell, and remixing said treated side stream with the main stream, before feeding them to the cooling tower.
According to a preferred embodiment of the invention the treated side stream leaving the electrolytic cell is further filtered to remove carried over solids. A preferred filter for this purpose is a sand filter with backwash, but of course other suitable filters will be recognized by the skilled engineer.
All the above and other characteristics and advantages of the invention will be better understood through the following illustrative and non-limitative description of preferred embodiments thereof.